This invention relates generally to ion detectors and more particularly to an electron capture type of detector for use in detecting the constituent gases of a sample eluted from a gas chromatograph.
Ionization detectors for gas chromatography are well known in the art and have been described in the literature. For example, a review of such detectors is contained in an article entitled "Ionization Methods for the Analysis of Gases and Vapors", by J. E. Lovelock, Analytical Chemistry, Vol. 33, No. 2, February 1961, pages 162-178. The most commonly used ionization detector in gas chromatography is the radioactive source electron capture detector. This type of detector features high sensitivity and high selectivity towards electrophilic compounds and is widely used for detecting trace amounts of pesticides in biological systems and in food products. Such compounds typically containing halogens combine with free electrons in the detector. The resulting decrease in free electrons is monitored and used as an indication of the concentration of the test substances in a sample.
Traditionally, the free electrons in an electron capture detector are produced by radioactive beta emitters in the form of foils or platings disposed inside the detector. Examples of such beta emitters are Tritium (H.sup.3) and Nickel-63 (Ni.sup.63).
There are disadvantages to the use of radioactive detectors. They may be operated only under license from a Federal and/or state governmental agency. Tritium detectors are prohibited from operating at temperatures above 325.degree. C. to avoid release of radioactivity into the atmosphere. Nickel-63 detectors must undergo periodic wipe tests to insure against radioactive leaks.
An additional disadvantage is that the radioactive beta emitters produce particles having sufficient energy to undesirably polymerize various compounds which may be present in the detector, thus necessitating frequent cleaning of the detector. Also, the energetic radicals created by beta bombardment can create a host of side reactions. Careful operation is often required to maintain operation of the detector in the electron capture mode rather than as a mobility or cross-section detector. Furthermore, in the case of a Nickel-63 detector, the betas emitted have a relatively long range so it is necessary to keep the volume of the detector relatively large, on the order of a milliliter, to avoid undesirable cross-section effects. Such a large volume is a serious drawback which precludes the detector from being used efficiently with capillary columns which elute small gas flows and therefore require a small volume detector.
Still another disadvantage of radioactive detectors is that the free electrons are created from relatively few primary particles. Consequently, the detector has significant shot noise which limits the minimum detectable quantity of a test substance.
Various attempts have been made to overcome the disadvantages of radioactive source electron capture detectors. For example, a radioactive source protected from contamination by use of a scavenging gas has been proposed by N. L. Gregory in the Journal of Chromatography, Vol. 13, 1964, pages 26-32. Other researchers have tried to produce free electrons in a detector by use of gaseous corona discharge, radiofrequency discharge, or photoionization techniques. Commercial use of such devices have been found unsatisfactory in electron capture detection because of high noise generation, source instability, lack of detector sensitivity, or a low signal-to-noise ratio.
The use of a non-radioactive, thermionic emitter as the source of free electrons has been proposed by A. K. Braude and V. A. Rotin. The emitter and other detector components are disposed in a common chamber, and a flow of guard gas is intended to reduce the contamination of the emitter by the test substance. The work of Braude and Rotin has been published in Chemical Abstracts, Vol. 80, 3363K, 1974, and in U.S.S.R. Pat. No. 375548 issued Mar. 23, 1973.